1. Field of the Invention
The present invention relates to process for making a gold salt to be used in electroplating processes; and more particularly to a process for making sodium gold sulfite.
2. Description of the Prior Art
Electroplating is a widely practiced technique for producing gold or gold alloy coatings for decorative or industrial purposes. Historically, the gold electroplating bath formulations have been primarily based on cyanide, predominantly the cyanide complex of monovalent gold, (Au(CN)2)xe2x88x92. Although these formulations have been successful, the use of extremely toxic cyanide ions is not desirable.
Electroplating solutions containing gold in the form of a soluble sulfite complex have been growing in use due to the fact that they avoid the more dangerous gold cyanide complexes. Gold sulfite baths are believed to provide better ductility and throwing power, a good alloy deposition, and a higher tolerance to impurities. Although these solutions have the advantage of being free from cyanide ions, the production of the old sulfite salt to be used in the electroplating solutions can be difficult, hazardous and expensive. Historically, the production of alkali metal gold sulfite salts utilizes the explosive gold fulminate as an intermediate. Gold fulminate is then added as a wet cake to a solution containing excess sodium sulfite to form the desired pale yellow sodium gold sulfite solution. Trace levels of the gold fulminate remain as a by-product such that acid salts prepared from the solutions can be hazardous to handle.
Gold sulfite salts in solution become unstable over time as a result of the oxidation of sulfite to sulfate. Therefore, it is desirable to convert the gold salt to the more stable solid form for transportation and storage purposes. It is also cheaper to transport the lighter solid salt than it is to transfer the solution containing water. One method for solidifying the solution involves evaporating the water from the solution. However, as the water evaporates to form the solid in these processes, it becomes explosive at temperatures of 75xc2x0 to 85xc2x0 C. If the evaporation of water is attempted at room temperature, the gold sulfite compound oxidizes and is unusable for electroplating. An alternative method for solidifying sodium gold sulfite solutions involves precipitating gold sulfite from the solution in order to increase the gold content of the product. However, as a result, the solidified form of the salt is chemically unstable, and the sulfite oxidizes to form sulfate, while the solid metal is reduced.
One of the major problems encountered when using gold sulfite solutions in gold electroplating processes is that the amount of gold plated per quantity of electricity applied decreases with time. This is due, in part, to the fact that oxygen from the atmosphere dissolves into the plating bath, oxidizing the sulfite to sulfate. The prior art suggests solving this problem by purging the solution with nitrogen or by using chemical oxygen scavengers to remove the dissolved oxygen. However, this technique may not always be practical since the physical set-up may not lend itself to nitrogen purging. Additionally, the gas and equipment are expensive, controls must be used, and the amount of agitation required is too large. Another solution suggested by the prior art is to use stabilizers, such as sodium thiocyanate. The disadvantage to using sodium thiocyanate is that the plating bath must be maintained at a pH of less than 7 in order to prevent the reduction of gold.
In the process of the present invention, a sodium gold sulfite solution is prepared from barium hydroxide and sodium hydroxide. This process affords the sodium gold sulfite as a concentrated solution in the absence of undesirable and dangerous by-products, such as gold fulminate. In another aspect of the invention, the sodium gold sulfite solution is solidified by a freeze-drying process. This freeze-drying process allows for the preparation of sodium gold sulfite in solid form which is oxidatively stable and therefore safer for transportation and storage. The solid form is also less expensive to ship since the water has been removed from the product. The invention also provides for a gold sulfite electroplating bath solution containing an alkali metal pyrophosphate salt which stabilizes the bath solution by preventing the oxidation of sulfite ions to sulfate ions. In another aspect of the invention, the waste gold from the sodium gold sulfite process is recycled as a starting material to provide a more cost effective production.